In the past I've enjoyed reconfiguring my empires slightly by designating a new sector capital, allowing it to mature, and then redesignating the original sector capital to be something else. This is a fairly painless way to extend control to a few useful nearby worlds that might have been missed during the initial sector capital establishment. A 250LY limit will put more pressure on players to pick absolutely flawless sector capital locations the first time, which is not really very fun (right now you have to manually measure the control radius with a ruler on your screen to see what your capital will be able to control.) With a 250LY limit, players will still be able to reconfigure but they will have to redesignate the existing sector capital and then wait 24 hours before regaining control of the region that it was servicing when the new capital comes on line. This is extremely dangerous, but maybe it's OK?

New players might not be able to figure out how to expand, since in F&M it's not necessary to build a second jumpship yard for quite a long time. Players may not realize that jumpship yards extend the range that jumpships can operate in beyond the beacon range of the imperial capital. Perhaps the beacon range for the imperial capital could be a little larger than normal, maybe 300LY?

Independent worlds with fated TLs 5 and over now have much, much stronger defenses. These are the only worlds that can become sector capitals if the limit gets pushed to 250LY, since it won't be possible to pre-raise future capitals through designation, foundations, etc.). The new sector capital planet will potentially be the strongest world that a player has ever attacked, especially if they've started in a bad or tight spot like a corner or rift zone region. A mixed colonization fleet going up against a strong world will run into problems where the transports get picked off by hypersonics, etc. before the escorts can destroy the defenses. The solution right now is probably to send two fleets, with the transports arriving a few watches after the first wave attacks with only a token gunship escort, but players likely won't figure this out the first time they try it and might get discouraged. This ties in with other players' requests that fleets should arrive at higher orbits and that transports should orbit higher.

T&E will not be able to establish links directly between sector capitals, although I personally don't care for the way that T&E makes sector capitals serve as hubs anyway. T&E can also build normal hubs so this isn't a huge issue.

None of these are issues with the 250LY limit itself, but rather with how players obtain information about and from the game. In practice >250LY is already a smart strategy for "real" (as opposed to spammed) capitals because it limits the impact of planet defections with sector capital capture.

I'll likely go ahead with the change and see how it feels in practice. And I agree that a lot of the problems can be ameliorated with better UI. E.g., it should be possible (somehow) to see sector capital range BEFORE designating.

One final caution: If there are current empires with sector capitals violating the limit, then they will be OK until the empire changes doctrine. At that point, some sector capitals may turn into autonomous to adhere to the limits. This is only a problem for existing empires, not for anyone going forward, so I think it's OK (and not worth writing extra code to prevent).

To address the first issue, of reconfiguring established empires, maybe only designating a capital from an independent world would be affected by the minimum? The idea being that for any world to not be independent, it must have been within range of an established capital, and so its designation is not an expansion technique.